A. Field of the Invention
The present invention relates generally to the field of electrical lighting devices and, more specifically, to fluorescent lamps which feature electronic ballast circuits for the control of multiple fluorescent lamp bulbs of various types and light output ratings.
B. Description of the Prior Art
The prior art devices used for illumination in a work area have included battery powered flashlights that have a limited life and a narrow focus; incandescent drop-lights that feature electrically inefficient, very hot and volatile tungsten filaments; and also various types of fluorescent lights. Battery-powered flashlights typically offer less illumination than AC powered lights, and the batteries, even if rechargeable, are not operable continuously without recharging or replacing the batteries.
Portable, hand-held drop lights or task lights utilizing an incandescent bulb and powered by AC line current, typically 120 Volts AC, 60 Hz, allow the user to provide light where installed light fixtures do not provide adequate coverage. However, incandescent bulbs as the light source in task lamps have several disadvantages. Although incandescent light bulbs are a well-developed technology and are economical to purchase, they are not economical to operate. Much of the electrical energy used by the task light is converted to heat. The tungsten filament in a typical 100 Watt incandescent bulb causes the bulb to get too hot to touch in many common work area situations such as a task lamp being used by a technician to illuminate the engine compartment of an automobile. Moreover, the relatively fragile nature of the incandescent lamp with its glass bulb and its tungsten filament presents further drawbacks.
One alternative to the use of incandescent bulbs is the fluorescent bulb. Fluorescent bulbs convert more of the supplied electrical energy to light energy and operate at lower external temperatures than do incandescent lights. The light emitting medium in fluorescent lights is a phosphor coating, unlike the thin, fragile tungsten filament in an incandescent light bulb. In a fluorescent lamp bulb, a glass tube containing a small amount of gas—mercury vapor, for example—is provided with coated cathode electrodes at either end of the tube. When a high enough voltage is applied between each pair of electrodes at the ends of the glass tube, the coated filament is heated and emits electrons into the gas inside the tube. The gas becomes partially ionized and undergoes a phase change to a plasma state. The plasma is conductive and permits an electric arc to be established between the electrodes. As current flows in the plasma, electrons collide with gas molecules, boosting the electrons to a higher energy level. This higher energy level is not a stable condition and when the electron falls back to its normal energy level, a photon of ultra-violet light is emitted. The photons in turn collide with the phosphor coating on the inside of the glass tube, imparting their energy to the phosphor ions, causing them to glow in the visible spectrum. Thus the phosphor coating luminesces and gives off the characteristic “fluorescent” light.
Fluorescent bulbs, however, typically require a higher voltage to initiate the plasma state than is required to maintain the plasma state and the luminescence. Further, after becoming a plasma, the effective resistance of the plasma between the electrodes drops increasingly as the current increases. Unless the current is limited, the tube will draw excessive current and damage itself and/or the supply circuit. Typical practice to limit the current is to provide a damping circuit, called a “ballast,” that operates to ignite the gas tube while also limiting the supply current. The ballast for full-sized installed light fixtures includes a large transformer/inductor, to transform the supplied line voltage, typically 120 Volts AC available at a wall outlet to a high enough potential to ignite the lamp and also to provide a high enough inductive impedance in the supply circuit to limit the current during operation. For typical installed lighting fixtures using non-self-starting bulbs and operating at 120 VAC, 60 Hz, the wire gauge, the number of turns in the coils, and size of the magnetic core result in a large and heavy ballast component. The ballast circuits for so-called “self-starting” fluorescent bulbs are typically smaller, yet still provide an appropriate voltage to ignite the lamps without a separate starter. The ballast circuit then regulates the current draw in a similar manner to that previously described for non-self starting bulbs.
Battery operated fluorescent lamps are well-known in the art but their use is usually limited to applications such as camp lighting or emergency lighting where a bright illumination is not essential. Battery operated fluorescent lamps typically cost more initially because they require an extra circuit to produce a high voltage, high frequency AC supply to operate the fluorescent bulb. Operation of the lamp at a higher frequency enables the use of smaller components which cost less and take up less space. Heretofore, a battery operated fluorescent task lamp, because of the additional circuit complexity, was relatively more expensive than a conventional AC line-operated task lamp. Such a ballast circuit thus can add complexity, cost, and an increased electrical load on the battery power supply if not carefully designed.
In one example of the prior art, U.S. Pat. No. 6,534,926, Miller et al., a portable fluorescent drop light is disclosed that contains a pair of twin-tube compact fluorescent lamp (CFL) bulbs that are individually switched. The discrete solid state drive circuit used as a ballast for non-self-starting bulbs utilizes the CFL bulbs as part of the oscillating circuit and has a relatively high component count. The circuit relies on several transformers, including separate windings for each CFL bulb, and a separate feedback protection circuit, to produce the drive voltage while limiting the current drawn by the bulbs during the run condition. Moreover, a different circuit is required for use with self-starting bulbs. Miller et al. thus has the disadvantages of relatively high component count, use of several transformers, and is not capable of driving non-self-starting or self-starting bulbs from the same ballast circuit. Further, since the lamps themselves are part of the oscillator circuits in Miller et al., the oscillator frequency is not independent of the variations in lamp characteristics unit-to-unit, and is thus subject to varying levels of performance.
A need exists, therefore, for an economical, portable hand-held task lamp that provides light output equivalent to that of a 100 Watt incandescent bulb, is efficient to operate, and does not operate at excessively high temperatures. A need also exists for a ballast circuit design which eliminates the need for a heavy transformer component and can preferably work with either self-starting or non-self-starting bulbs. The lamp should be sturdy and durable and replacement bulbs should be inexpensive, readily available, and easily changed. It would be a further desirable feature to provide variable illumination and as light-weight and compact a lamp as possible.